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Aerosols

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Title: Aerosols


1
Coast Day
College of Marine Studies
October 4, 1998
Atmospheric and Oceanic Aerosols
and Climate
Magdalena AnguelovaPh.D. Student
Advisor Prof. Ferris Webster
Duration 45 min.
2
A sunrise over the China Sea
This photograph is taken by the crew of the
Space Shuttle.
Here the black shadows against the sunlit
horizon are high-peaking clouds. The colorful
bands above are atmospheric layers and their
exceptional brightness is due to concentration of
dust in the atmosphere. Dust and other types of
particles, called aerosols, and their effect on
climate are the subject of this poster.
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3
Outline
  • The big picture - climate elements (18 screens)
  • What are aerosols? (2)
  • Why are aerosols so important? (3)
  • Aerosol properties (3)
  • Aerosol types (3)
  • Aerosol sources and formation (12)
  • Global distribution (5)
  • Summary
  • Hypothesis

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4
The big picture Sun, Earth, and Atmosphere
The climate system on our planet is driven
by the energy coming from the sun. The sunlight
reaches the Earth through several atmospheric
layers.
The lowest one, from the Earth surface to
about 7miles height, is called troposphere.
Thermosphere
Mesosphere
Next layer, from 7 up to 30 miles above the
surface, is called stratosphere.
Stratosphere
Stratosphere
Mesosphere and thermosphere follow above
up to about 50 miles height.
Troposphere
Troposphere
The layers of interest for us are those
where the aerosols reside
the troposphere
and stratosphere.
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5
The big picture Sun spectrum
Recall each body with some temperature emits
radiation. We feel the
radiation emitted from our bodies as heat.
This law applies to all objects in the
Universe.
The sun is a celestial body with a
temperature of 6000 oC. Objects with such high
temperature emit energy at the so called short
wavelengths of the electromagnetic spectrum
visualized like this
The Sun emission peaks in the visible range.
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6
The big picture Earth Spectrum
In contrast, the Earth is colder celestial
body with average surface temperature of 15oC.
That is why Earth emits at longer
wavelengths, called infrared (IR), visualized
like this
long
short
Get oriented in Electromagnetic spectrum !
X-rays in medicine
Radio broadcasting
So, remember
The Sun emits at short wavelengths (SW).
The Earth emits at long wavelengths (LW).
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7
The big picture Solar energy at sea
level
Only a part of the SW solar radiation
available at the top of the atmosphere reaches
the Earth.
Some of it is scattered, absorbed and
reflected within the atmosphere by the gases,
aerosols, and clouds.
The absorbed radiation is re-emitted by the
atmospheric constituents back as a LW radiation,
i.e., it is converted in heat.
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8
The big picture Greenhouse
effect
Similar process takes place at the surface of
Earth
from the SW
solar radiation (A)
left at the sea level, part is absorbed (B)
and then re-emitted back (C)
to the atmosphere as LW IR radiation.
If there were no atmosphere, the IR
radiation emitted by Earth would escape to the
space and the planet would cool down.
But in presence of atmosphere, some IR
radiation is trapped and re-emitted back (D)
to Earth by naturally occurring gases as
CO2 , H2O vapors, and CH4.
This is the so called natural greenhouse
effect which keeps the Earths surface about 33
oC warmer than it would be if greenhouse effect
were not present.
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9
The big picture Climate System
How fortunate for all living creatures !
For the natural greenhouse effect makes our
planet habitable. Otherwise the Earth would be a
frigid and inhospitable place.
The atmosphere sets the greenhouse effect at
work, and the climate system is created. The
solar radiation powers it.
The climate machine does not stop if
something goes wrong in it. If small
perturbation in one of the elements appears,
e.g., a change in solar emission, or a change in
ocean shapes due to plate tectonics, the system
tries to readjust to the new conditions.
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10
The big picture Climate System Elements
The main elements of the climate system are
the atmosphere and the oceans.
The fast heating and cooling of land,
the strong reflection of the sunlight by ice and
snow,
the clouds
and precipitation are the other elements of this
machine.
Their interaction with the basic elements
makes the last touches in this almost perfect
harmony.
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11
The big picture How does it work?
The warmth of the Sun is not distributed
uniformly over the globe. It is maximum at the
equator and the tropics
and minimum at the polar regions.
The climate machine churns relentlessly
attempting to smooth out this temperature
imbalance by cooling the tropics and warming the
poles.
The wind system and ocean currents do the work.
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12
The big picture Winds and Currents
Unequal heating of the atmosphere sets up
convection - rising of warm air at the tropics
and sinking of cold air at high latitudes.
This in turn sets a regular system of winds
called Trades (or Easterlies)
and Westerlies.
These, together with frontal storms,
transfer cold air equatorward and warm air
poleward.
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13
The big picture Winds and Currents
The winds drag the water in the oceans and
form a system of immense ocean currents.
They transport cold water toward the Equator
and warm water to the poles.
As a result of this heat transfer, the
average temperature anywhere on the Earth is
quite stable over long time period.
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14
The big picture Radiation
Budget
So, on a long-time scale, the climate system
is in equilibrium. This usually is demonstrated
with the radiation budget of the planet. Let see
? The SW radiation coming to the Earth is 340
W m-2.
? About 30 of it is directly reflected back
to the space.
? The remaining 240 W m-2 are absorbed by the
Earth-atmosphere system.
? As the law requires, the same amount is
emitted as LW radiation back to space.
Absorbed SW 240 W m-2
This is the natural and
necessary balance !
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15
The big picture Troubled Radiation
Budget
We, humans, are adding more and more
greenhouse gases into the atmosphere by burning
fossil fuels.
The trouble is...
Even worse, we increase not only the
concentration of the naturally occurring
greenhouse gases, but add unnatural greenhouse
gases, such as nitrous oxide (N2O) and
chlorofluorocarbons (CFCs).
In addition, we cut down thousands of trees
for lumber, making them unable to take CO2 out
of the air.
All this waste in the air is letting less
and less heat to go back to space. And, the
more CO2 and other greenhouse gasses in the
atmosphere, the more IR radiation is trapped and
re-emitted back to the Earth, the warmer it
becomes with possible catastrophic effects.
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16
The big picture Global
Warming
This greenhouse effect in excess of the natural
one is termed global warming. Scientists try to
model and predict the effect of global warming.
They recalculate the radiation budget with
increased concentration of CO2.
If the amount of CO2 doubles, the outgoing LW
radiation would decrease by 4 W m-2.
This imbalance would induce a gradual change in
order to restore the amount of leaving radiation
from 236 back to 240 W m-2. This would require
an increase in global mean surface temperature by
1.2 K.
And this is a trouble !
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17
The big picture Modeling Global
Warming
The current models, however, produce both
greater warming and substantial disagreement
from 1.7 to 5.4 K.
The main reason for the disagreement stems from
the different depiction of the climate feedback
mechanisms in the models. These can either
amplify or moderate the warming.
E.g., a warmer climate means a warmer atmosphere
with more water vapor, which itself is a
greenhouse gas. So, water vapor provides a
positive (or amplifying) feedback mechanism.
Different models generally consent on this
particular feedback.
The feedback associated with cloudiness,
however, turns out to be much more difficult
matter.
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18
The big picture Radiation Budget
Without Clouds
Let track the radiation budget of a
hypothetical planet with the same surface
temperature but without clouds
? The same coming solar radiation
? In absence of clouds less SW radiation is
reflected back to space, only 50 W m-2 instead of
100 W m-2
? Earth-atmosphere system absorbs the
remaining 290 W m-2, instead of 240 W m-2
? At 15 degrees surface temperature
Earth-atmosphere system emits only 270 W m-2
Absorbed SW 290 W m-2
and ...
There is surplus of 20 W m-2 !
Obviously, the clouds balance the system. How?
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19
The big picture Cloud
Feedback
The effect of clouds on the Earth-atmosphere
system is termed as Cloud-Radiative Forcing
(CRF).
We see, the clouds enhance the SW reflection
and cool the system by 50 W m-2. In this way the
clouds exert negative feedback.
But they also absorb LW radiation coming
from the earth and re-emit it back. So that
simultaneously with the negative the clouds
provide also positive feedback and warm the Earth
with 30 W m-2.
The net result of these two opposite
processes is cooling by 20 W m-2.
So, Cloud Radiative Forcing (CRF)
closes the balance of absorbed and emitted
radiation.
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20
The big picture Modeling Cloud
Feedback
How exciting !
The cooling by clouds would mitigate the global
warming !
However, the cooling by clouds may change as
the climate changes due to global warming.
Scientists constructed and ran models again to
see how the cloud radiative forcing would change.
Here 19 (!) different models show quite
different results for cloud feedback
from modest cooling
through almost missing
to strong positive.
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21
The big picture What about Aerosols?
The discrepancies in modeling the cloud feedback
pointed out that we need to know well the cloud
properties and their global pattern. So, since
the beginning of 90s studies of clouds have
priority and many programs for measuring the
global cloud coverage and properties have been
initiated.
You probably ask yourself already impatiently
Where in this long story are the aerosols?
Well, the findings about clouds gradually
showed that the cloud properties and lifetime are
significantly affected by aerosols.
How exactly? It turned out we do not have
enough knowledge about aerosols in order to know
how they do that.
So, for the last 2 years the aerosols, not
the clouds, are the Gordian knot of the climatic
studies.
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22
What are Aerosols?
Aerosols are minute stable particles, solid
or liquid, suspended in the atmosphere.
Samples of clean (rural) and polluted
(urban) air under microscope show
different particle shapes (spherical or
arbitrary) and concentrations.
For the scale 1 ?m 10-6 m
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23
We see Aerosols as...
Aerosols are too small to be observed by
naked eye.
We do not see the air molecules too. But we
see the result of scattering of the sunlight by
them as blue sky.
Similarly, the red sunsets and sunrises are
result of scattering and absorbing of the
sunlight by aerosols.
The red color comes from the fact that the
aerosols are larger than the air molecules and
scatter more effectively the red light than the
blue one.
Another manifestation of the presence of
aerosols is haze.
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24
Why are Aerosols so Important?
Atmospheric aerosols influence the climate
in two ways
? directly - through the reflection and
absorption of solar radiation. The
mechanisms are well understood
? scattering of coming SW solar radiation back
to space
? absorption of coming SW solar radiation.
In both cases less radiation reaches and
heats the Earth, i.e., the aerosols cool the
Earth-atmosphere system.
? indirectly - through modifying the optical
properties and lifetime of clouds.
Aerosols act as cloud condensation nuclei
(CCN) on which H2O vapors in the atmosphere
condense and form cloud droplets. Two scenarios
are at work
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25
Cloud/Aerosol Scenarios
When there are more aerosols (i.e., more
CCN), more droplets form in the cloud. We
observe
1) more surface available to reflect the light.
Net result cloud albedo (reflection) increases.
2) inhibition of the growth of the existing
droplets, hence condensation and rain are
delayed.
Net result prolonged cloud lifetime.
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26
More Roles for Aerosols
Aerosols act as sites for chemical reactions
to take place.
The most significant example destruction
of stratospheric ozone.
During winter in the polar regions, aerosols
grow to form polar stratospheric clouds.
The cloud particles provide huge surface area
for chemical reactions.
These reactions lead to the formation of large
amount of reactive chlorine, which ultimately
leads to destruction of the ozone in the
stratosphere.
Increased aerosol pollution from 1979
to 1989
resulted in ozone hole over Antarctic.
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27
Aerosol Properties
The effect of aerosols on climate is termed
aerosol radiative forcing.
To estimate aerosol radiative forcing we
need to know which aerosol properties control the
different processes.
Aerosols represent only a small part of the
mass of the atmosphere. Yet, they have the
potential to influence the heat budget of the
planet. The reason is that most processes
involving aerosols are controlled by the aerosol
surface, not by the aerosol mass.
That is, many small particles do better than few
large.
Thus, the most important aerosol properties are
? size and shape
? concentration
and
? lifetime
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28
Aerosol properties Size and Shape
Typical distribution of aerosol mass and number
by size.
coarse
We see 2 peaks over
and fine particles.
The size controls the physical and chemical
processes !
Particles with diameter 0.01 - 0.05 mm act as
cloud nuclei
0.08 to 0.5 mm accumulate mass
0.1 - 2 mm efficiently scatter the light
above 1 mm provide medium for chemical
reactions.
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29
Aerosol properties Concentration and
Lifetime
Concentration is defined as the total number
of particles per unit volume
Concentration changes with height and site, being
higher where the aerosols form
look - there are more aerosols close to the
Earth surface, and their number decreases with
height
there are more aerosols close to the
continents, and their number decreases in remote
oceanic places.
Lifetime is the time aerosols reside in the
atmosphere before being removed by precipitation
or conversion in something else
Aerosol lifetime ranges from 2-3 years to 3-5
days
Most aerosols are short-lived.
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30
Aerosol Types
There are many types of aerosols
classified by different criteria.
Depending on their size aerosols are (we
already know this) coarse and fine
Depending on their source aerosols are
natural - produced by volcanic emission or
oceans anthropogenic - result of the human
activities
Depending on their mechanism of formation
aerosols are primary - delivered to the
atmosphere directly as particles secondary
- formed within the atmosphere from gases
Depending on their residence site aerosols
are tropospheric and stratospheric
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Aerosol Types Natural Aerosols
Examples of natural aerosols are
Primary (formed directly as particles)
Most numerous are
Soil dust (mineral aerosol)
Secondary (formed in the atmosphere from gases)
Sea salt
Volcanic dust
Organic aerosols
Sulfates from biogenic gases
Sulfates from volcanic SO2
Organic matter from biogenic C
Nitrates from NOx
Let talk about some of these !
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Natural Aerosols Soil
Dust Sources
Major sources of soil dust are arid regions
such as deserts of Northern Africa and Asia. One
of the largest source is Sahara desert.
This photograph is a good example We see a dust
storm north of Arabian
Sea - a basin surrounded by arid terrain.
The area joining Iran,
Afghanistan and Pakistan
experiences the highest frequency of
dust storms in the world over 30 dust storms per
year.
Imagine, this village endures such a mess twice,
sometimes more, a month !
As a result,
the deposition rate of mineral aerosols in the
Indian Ocean is more than 5 times any other
region of the world oceans.
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Natural Aerosols Formation
Soil Dust Lifting
The number of aerosols delivered by extreme
events as dust storms dominates the number of
aerosols created by continuous lower wind.
Though the particles produced in this way
are relatively large, they are found all over the
globe the strong winds lift them at high
altitudes and the atmospheric circulation
transports them over thousands of kilometers.
There are 4 mechanisms of detachment and lifting
of soil particles by wind
(a) creeping - one large particle bounces several
times creating many smaller particles
(b) turbulent lifting - strong wind projects
particles directly in air
(c) surface collision
(d) soil splashing.
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Natural Aerosol Source of Sea Salt
Aerosols
On a windy day, when even a skillful
Hawaiian surfer may flip, the ocean is covered
with whitecaps. Whitecaps are the major source
for sea salt particles. They produce numerous
drops, which evaporate, shrink to a smaller size,
and form sea salt aerosols.
Interestingly enough, the vast oceans produce
a bit less aerosols than deserts produce dust.
The reason is that the formation of sea
salt drops, parenting the sea salt aerosols,
includes several processes requiring more energy
than mere lifting of a dust particle.
Let see these processes.
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Natural Aerosol Formation
Sea Salt
Air entrained into the water after wave breaking
creates bubble clouds.
The large bubbles rise to the surface and burst.
Their caps shatter in thousands small droplets
called film drops.
Upon bursting, bubble cavity collapses, a water
jet rises from its bottom, and several small
drops, called jet drops, are pinched from the
tip.
Under high winds so called spume drops are torn
from the crests of breaking waves and blown
directly into the air.
Least drops are formed by splashing mechanism,
when some small unstable projections of water
form drops.
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Natural Aerosol Volcanic dust
This photograph of the eruption of Mt. St.
Helens in 1980 is a good example for the huge
clouds of ash particles and gases, including
sulfur dioxide, that volcanoes blast into the
atmosphere as they erupt.
Short-term global cooling often has been
linked with such events.
The year 1816 has been referred to as the
year without a summer. It was a time of
significant weather disruption in New England and
in Western Europe with killing summer frosts in
the United States and Canada.
The unusual weather was attributed to a major
eruption of the Tambora volcano in 1815 in
Indonesia.
The volcano threw sulfur dioxide gas into the
stratosphere, and the aerosol layer that formed
led to brilliant sunsets seen around the world
for several years.
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Natural Aerosol Cooling by Volcanic Dust
Aerosols in atmosphere increase after major
eruptions
The relative global cooling of 1993 is
ascribed to the eruption of Mount Pinatubo in
1991. Several weeks after spreading of volcanic
dust across the Pacific, the sulfur dioxide had
spread all over the world.
Not all large volcanic eruptions produce
global-scale cooling. Mount Agung in 1963 caused
a considerable decrease in temperatures around
much of the world, whereas El Chichón in 1982
seemed to have little effect.
The red color shows maximum aerosol concentration
It is believed the 1982 El Niño cancelled
out the effect of the El Chichón eruption.
SO2 cloud from Mt. Pinatubo, September 23, 1991
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Natural Aerosol Formation
Volcanic dust
Millions of tons of ash and SO2 gas can reach
the stratosphere from a major volcano.
The ash is soon washed out by rain.
SO2 stays and under the action of light
converts to tiny aerosols of sulfuric acid.
These aerosols are persistent, and after the
stratospheric winds spread them over the globe,
they stay there for several years.
These particles reflect the sunlight, thereby
cooling the Earth.
They grow slowly and are regularly removed by
rain for a long time.
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Natural Aerosols Sulfates and organic matter
The living creatures in the oceans and on
land are involved in the creation of organic
aerosols and sulfates.
Bubble bursting in oceans and burning of
terrestrial vegetation deliver organic carbon
and other particles.
Phytoplankton in the oceans emits gas called
Dimethylsulphide (DMS). DMS is transferred into
the atmosphere where organic aerosols form by
gas-to-particle conversion.
Emissions of natural organic aerosols from
oceans dominate the terrestrial sources.
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Aerosol types Anthropogenic Aerosols
Examples of anthropogenic aerosols are
Primary (formed directly as particles)
Most numerous are
Industrial aerosols
Biomass burning
Secondary (formed in the atmosphere from gases)
Soot
Sulfates from industrial SO2
Organic matter from biogenic C
Nitrates from NOx
Let talk about some of these !
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Anthropogenic Aerosols Sources of Industrial
Pollution
The primary industrial aerosols originate
from inorganic impurities in the fuel we use or
from incomplete fuel combustion.
The primary industrial aerosols originate
from inorganic impurities in the fuel we use or
from incomplete fuel combustion.
  • All these sources increase
  • carbon dioxide (CO2)
  • methane (CH4)
  • nitrous oxide (N2 O)
  • halocarbons (CFCs)

Airplanes and factories release water vapor
forming additional clouds and reflecting the
incoming sunlight.
Airplanes and factories release water vapor
forming additional clouds and reflecting the
incoming sunlight.
Most dangerous are the sulfate aerosols
formed from these gases, because they are
persistent.
About 90 of the sulfur emissions are from
industrial regions in the Northern hemisphere.
Recent estimates show that contrails are able to
change the climate locally in regions with heavy
airplane traffic.
Recent estimates show that contrails are able to
change the climate locally in regions with heavy
airplane traffic.
Plants, cars, and aircraft emit directly soot,
and nitrogen oxides.
Plants, cars, and aircraft emit directly soot,
and nitrogen oxides.
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Anthropogenic AerosolsBiomass Burning - natural
and...
Biomass burning refers to the burning of the
world's forests, grasslands, and agricultural
lands. It releases significant quantities of
gases and particles into the atmosphere.
There are natural fires like this one in
Arizona, but it is generally believed that most
biomass burning is human-initiated.
The oil fires in Kuwait is one such example.
The Hochderffer fire, Coconino National Forest,
AR
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Anthropogenic Aerosols Man-initiated
Biomass Burning
The biomass burning has increased
significantly over the last century. Regular
measurements and monitoring from space helped in
the last few years to understand that biomass
burning is much more widespread than previously
thought.
Biomass burning is a widespread practice for
land clearing and land use change such as
conversion of forest regions to grazing and
agriculture areas.
Roughly 175 million acres of forest and
grassland are burned each year world-wide.
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Anthropogenic Aerosols Formation
Biomass Burning
Combustion gases include CO2, CO,
hydrocarbons, NxO, etc. CO2 and CH4 are direct
addition to the greenhouse gases. The other gases
are chemically active and impact the composition
and chemistry of the troposphere, leading to
destruction of ozone.
80 of the total biomass burning occurs
in tropical rain forests and savanna grasslands
2/3 of the Earth's savannas are located in
Africa, recognized as the "burning center" of the
planet.
Biomass burning extends to fire-free
regions as smoke and aerosol particles rise high
into the troposphere and are carried long
distances by winds.
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Aerosol global distribution
Satellite observations reveal that there is
no "global aerosol" that fills the troposphere
with a uniform background aerosol.
The global aerosol distribution is a
collection of independent aerosol regions each
having its own source and unique spatial temporal
pattern.
Marine aerosols dominate large areas, but
continental aerosol plumes show more intense
reflection of sunlight. Hence, the aerosol impact
over the continents is likely to be much higher
than over the oceans.
The aerosol reflection is strongest in the
Tropics where most of the solar radiation is
absorbed and aerosol-cloud interactions are
intense.
There is a pronounced seasonality in each
aerosol region the higher aerosol levels appear
in the summer.
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Aerosol global distribution Oceanic
aerosol
Indeed, aerosols are concentrated in the
Tropics and their reflection is higher in summer
than in winter.
Even more aerosols are present during
phytoplankton bloom in spring.
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Aerosol global distribution Oceanic
CloudCondensationNuclei
Recall the more aerosols, the more nuclei for
forming cloud drops (CCN).
We see, most CCN are around the continents
where the aerosols produced by human activity are
most.
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Aerosol global distribution Continental
aerosol over oceans
Once again the same seasonal pattern more
aerosols in summer than in winter.
Note 2 places we considered
The pronounced plume from Africa - Sahara and
savanna fires produce enormous quantity of
aerosols
Indian ocean - the arid areas around Arabian
sea with strong dust storms.
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Aerosol global distribution
Volcanic aerosols
This is animation showing the spreading of
aerosols after 3 volcano eruptions in period 1985
- 1997 rate - every 3 months.
Red high aerosol reflection.
Eruptions Nevado del Ruiz, Columbia, 1985
Most of the volcanic aerosols were high in the
stratosphere and remained obvious for several
years.
Kelut, Indonesia, February, 1990 small increase
Mt. Pinatubo, 1991 the dominant event in this
animation, aerosols in stratosphere increased by
a factor of 30.
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Aerosol global distribution
Sulfur emissions
As expected, the industrial regions are the
major sources of anthropogenic sulfate aerosols.
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Summary
  • Aerosols influence the climate
  • ? directly via scattering of sunlight
  • ? indirectly via changing clouds optical
    properties

Aerosols provide medium for chemical
reactions in the atmosphere
Aerosols are unevenly distributed over the
globe
Aerosols are short lived with exception of
the volcanic dust
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Hypothesis
There is a hypothesis aerosol cooling, mainly
due to man-produced sulfates, may cancel the
effect of global warming.
Calming but not yet proven idea
While uniformly distributed greenhouse gases
over the globe may cause global warming, the
uneven aerosol distribution may only cool places
here and there. This may still be not enough to
outweigh the warming.
We have much more work to do ...
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